Several types of switch mode converters and inverters are currently widely used for DC-to-DC, DC-to-AC, AC-to-DC and AC-to-AC power conversion. Certain loads need special drive signals in order to optimized their performance and maximize the system's efficiency. For example, fluorescent lamps need to be driven by a source that has a high output impedance to stabilize their operating point, and by high frequency sign to increase their light output. Accordingly, an inverter driving a fluorescent lamp must have a current source nature (as opposed to voltage source characteristic), such that the magnitude of its output current is relatively independent of the resistance of the lamp. One way to implement a high output impedance driver is to apply an electromagnetic ballast that is based on a large inductor, which is placed in series with the power line voltage. An alternative and preferred approach is to generate a high frequency signal by a switch mode inverter since the efficacy of the lamp increases when driven by a high frequency current. A typical prior art solution is illustrated in FIG. 1.
FIG. 1 depicts a line rectifier (1), a power factor correction section (PFC), a half-bridge inverter (Q1, Q2) that is operated by a gate drive and a controller that generates the gate signals. The requirement for a high impedance drive is accomplished by placing an inductor (Ls) in series with the lamp. At high frequency, the inductor exhibits a relatively high impedance that controls the magnitude of the current that is fed to the lamp. The purpose of the capacitor Cs is DC decoupling while Cp is used to generate a high voltage needed for igniting the lamp. The high peak voltage that is required for igniting the lamp is generated by driving the inverter during the start up stage, by a frequency that is close to the resonant frequency formed by the resonant network Ls, Cp. As a result, the voltage across the resonant capacitor Cp will build up to help ignite the lamp. The effect of the blocking capacitor Cs on the resonant frequency is small since its capacitance is much larger than that of Cp. Once the lamp ignites, the drive frequency is changed to the normal operating frequency. The electronic ballast of FIG. 1 can be used to drive a number of lamps. Each lamp will have to have its interface network including Cs, Ls and Cp.
The commercial realization of the electronic ballast of FIG. 1 is rather costly since there is a need for a dedicated controller as well as a gate driver for both the high side and low side switches of the half bridge. Furthermore, in a multi lamp configuration there is a need to add an interface network (Ls, Cs, Cp) for each lamp. A lower cost solution that is based on a self-oscillating inverter is depicted in FIG. 2. In this prior art approach, the circuit acts as an oscillator, generating its own drive signals to the gates of the power MOSFET switches. Consequently the Bill Of Material (BOM) of this ballast is far lower than that of FIG. 1. However, the driver of FIG. 2 needs to be designed for each particular power level. For example, L. S. Nerone (“A mathematical model of the class D converter for compact fluorescent lamps,” IEEE Transactions on Power Electronics, Vol. 10, No. 6, 708-715, 1995) teaches how to select the components of the self-oscillating driver for a given (single) lamp. R. K. Pavao et al. (R. K. Pavao, F. E. Bisogno, A. R. Seidel, R. N. do Prado, “Self-oscillating electronic ballast design based on the point of view of control system,” Industry Applications Conference, 2001. Thirty-Sixth IAS Annual Meeting. Conference Record of the 2001 IEEE , Vol. 1, 211-217 , 2001) describes a self-oscillating ballast for a single fluorescent lamp. The drawback of the self oscillating ballast, according to the design known in the art (FIG. 2), is that it is specific to a single lamp, requiring a ballast per lamp. Since, however, many applications call for multi-lamp fixtures, it would be highly advantageous to have a self-oscillating fluorescent lamp driver that can operate a number of lamps. This will reduce dramatically the cost and will increase the reliability of the system. Furthermore, it would be also highly advantageous if the same fluorescent lamp driver can operate one lamp or more. This will lower the number of models that need to be manufactured and stocked, decreasing thereby the production, storage and distribution cost of the product. It will be further be highly advantageous if the same driver circuit will be able to operate a number of lamps with no need to add component per lamp. It will also be advantageous if the efficiency of the driver is kept high when operating one, two or any number of lamps (up to some practical limit).
All of the methods described above have not yet provided a simple and economic way for constructing a self-oscillating fluorescent lamp driver that can ignite and operate one or more lamps while maintaining low cost and high efficiency.
It is the objective of the present invention to provide a method and apparatus for a self-oscillating inverter to produce a high frequency current that will be constant and independent of the load i.e. the number of lamps operated by it (up to some practical limit).
It is another objective of the present invention to provide a method and apparatus to maintain high efficiency of a fluorescent lamp driver when operating a single or a number of lamps.
It is yet another objective of the present invention to provide a sufficiently high ignition voltage that will ignite the lamps connected to the driver.
It is still another object of the present invention to extend the reliability of fluorescent lamp drivers.
Other objects and advantages of the invention will become apparent as the description proceeds.